tun: use correct IP header comparisons in tcpGRO() and tcpPacketsCanCoalesce()

tcpGRO() was using an incorrect IPv4 more fragments bit mask.

tcpPacketsCanCoalesce() was not distinguishing tcp6 from tcp4, and TTL
values were not compared. TTL values should be equal at the IP layer,
otherwise the packets should not coalesce. This tracks with the kernel.

Reviewed-by: Denton Gentry <dgentry@tailscale.com>
Signed-off-by: Jordan Whited <jordan@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

1 files changed, 94 insertions, 6 deletions

diff --git a/tun/tcp_offload_linux_test.go b/tun/tcp_offload_linux_test.go
index 046e177..9160e18 100644
--- a/tun/tcp_offload_linux_test.go
+++ b/tun/tcp_offload_linux_test.go
@@ -28,19 +28,23 @@ var (
 	ip6PortC = netip.MustParseAddrPort("[2001:db8::3]:1")
 )
 
-func tcp4Packet(srcIPPort, dstIPPort netip.AddrPort, flags header.TCPFlags, segmentSize, seq uint32) []byte {
+func tcp4PacketMutateIPFields(srcIPPort, dstIPPort netip.AddrPort, flags header.TCPFlags, segmentSize, seq uint32, ipFn func(*header.IPv4Fields)) []byte {
 	totalLen := 40 + segmentSize
 	b := make([]byte, offset+int(totalLen), 65535)
 	ipv4H := header.IPv4(b[offset:])
 	srcAs4 := srcIPPort.Addr().As4()
 	dstAs4 := dstIPPort.Addr().As4()
-	ipv4H.Encode(&header.IPv4Fields{
+	ipFields := &header.IPv4Fields{
 		SrcAddr:     tcpip.Address(srcAs4[:]),
 		DstAddr:     tcpip.Address(dstAs4[:]),
 		Protocol:    unix.IPPROTO_TCP,
 		TTL:         64,
 		TotalLength: uint16(totalLen),
-	})
+	}
+	if ipFn != nil {
+		ipFn(ipFields)
+	}
+	ipv4H.Encode(ipFields)
 	tcpH := header.TCP(b[offset+20:])
 	tcpH.Encode(&header.TCPFields{
 		SrcPort:    srcIPPort.Port(),
@@ -57,19 +61,27 @@ func tcp4Packet(srcIPPort, dstIPPort netip.AddrPort, flags header.TCPFlags, segm
 	return b
 }
 
-func tcp6Packet(srcIPPort, dstIPPort netip.AddrPort, flags header.TCPFlags, segmentSize, seq uint32) []byte {
+func tcp4Packet(srcIPPort, dstIPPort netip.AddrPort, flags header.TCPFlags, segmentSize, seq uint32) []byte {
+	return tcp4PacketMutateIPFields(srcIPPort, dstIPPort, flags, segmentSize, seq, nil)
+}
+
+func tcp6PacketMutateIPFields(srcIPPort, dstIPPort netip.AddrPort, flags header.TCPFlags, segmentSize, seq uint32, ipFn func(*header.IPv6Fields)) []byte {
 	totalLen := 60 + segmentSize
 	b := make([]byte, offset+int(totalLen), 65535)
 	ipv6H := header.IPv6(b[offset:])
 	srcAs16 := srcIPPort.Addr().As16()
 	dstAs16 := dstIPPort.Addr().As16()
-	ipv6H.Encode(&header.IPv6Fields{
+	ipFields := &header.IPv6Fields{
 		SrcAddr:           tcpip.Address(srcAs16[:]),
 		DstAddr:           tcpip.Address(dstAs16[:]),
 		TransportProtocol: unix.IPPROTO_TCP,
 		HopLimit:          64,
 		PayloadLength:     uint16(segmentSize + 20),
-	})
+	}
+	if ipFn != nil {
+		ipFn(ipFields)
+	}
+	ipv6H.Encode(ipFields)
 	tcpH := header.TCP(b[offset+40:])
 	tcpH.Encode(&header.TCPFields{
 		SrcPort:    srcIPPort.Port(),
@@ -85,6 +97,10 @@ func tcp6Packet(srcIPPort, dstIPPort netip.AddrPort, flags header.TCPFlags, segm
 	return b
 }
 
+func tcp6Packet(srcIPPort, dstIPPort netip.AddrPort, flags header.TCPFlags, segmentSize, seq uint32) []byte {
+	return tcp6PacketMutateIPFields(srcIPPort, dstIPPort, flags, segmentSize, seq, nil)
+}
+
 func Test_handleVirtioRead(t *testing.T) {
 	tests := []struct {
 		name     string
@@ -245,6 +261,78 @@ func Test_handleGRO(t *testing.T) {
 			[]int{340},
 			false,
 		},
+		{
+			"tcp4 unequal TTL",
+			[][]byte{
+				tcp4Packet(ip4PortA, ip4PortB, header.TCPFlagAck, 100, 1),
+				tcp4PacketMutateIPFields(ip4PortA, ip4PortB, header.TCPFlagAck, 100, 101, func(fields *header.IPv4Fields) {
+					fields.TTL++
+				}),
+			},
+			[]int{0, 1},
+			[]int{140, 140},
+			false,
+		},
+		{
+			"tcp4 unequal ToS",
+			[][]byte{
+				tcp4Packet(ip4PortA, ip4PortB, header.TCPFlagAck, 100, 1),
+				tcp4PacketMutateIPFields(ip4PortA, ip4PortB, header.TCPFlagAck, 100, 101, func(fields *header.IPv4Fields) {
+					fields.TOS++
+				}),
+			},
+			[]int{0, 1},
+			[]int{140, 140},
+			false,
+		},
+		{
+			"tcp4 unequal flags more fragments set",
+			[][]byte{
+				tcp4Packet(ip4PortA, ip4PortB, header.TCPFlagAck, 100, 1),
+				tcp4PacketMutateIPFields(ip4PortA, ip4PortB, header.TCPFlagAck, 100, 101, func(fields *header.IPv4Fields) {
+					fields.Flags = 1
+				}),
+			},
+			[]int{0, 1},
+			[]int{140, 140},
+			false,
+		},
+		{
+			"tcp4 unequal flags DF set",
+			[][]byte{
+				tcp4Packet(ip4PortA, ip4PortB, header.TCPFlagAck, 100, 1),
+				tcp4PacketMutateIPFields(ip4PortA, ip4PortB, header.TCPFlagAck, 100, 101, func(fields *header.IPv4Fields) {
+					fields.Flags = 2
+				}),
+			},
+			[]int{0, 1},
+			[]int{140, 140},
+			false,
+		},
+		{
+			"tcp6 unequal hop limit",
+			[][]byte{
+				tcp6Packet(ip6PortA, ip6PortB, header.TCPFlagAck, 100, 1),
+				tcp6PacketMutateIPFields(ip6PortA, ip6PortB, header.TCPFlagAck, 100, 101, func(fields *header.IPv6Fields) {
+					fields.HopLimit++
+				}),
+			},
+			[]int{0, 1},
+			[]int{160, 160},
+			false,
+		},
+		{
+			"tcp6 unequal traffic class",
+			[][]byte{
+				tcp6Packet(ip6PortA, ip6PortB, header.TCPFlagAck, 100, 1),
+				tcp6PacketMutateIPFields(ip6PortA, ip6PortB, header.TCPFlagAck, 100, 101, func(fields *header.IPv6Fields) {
+					fields.TrafficClass++
+				}),
+			},
+			[]int{0, 1},
+			[]int{160, 160},
+			false,
+		},
 	}
 
 	for _, tt := range tests {